This invention relates to a digital interface apparatus, and more particularly, to an apparatus for providing multiple controller interfaces to a standard digital modem, and includes a multiplexed method of resolving contention among the connected controllers.
In present communication systems utilizing a bus approach, which in particular conforms to IEEE standard 802.4-1989 section 10 (which is now also ISO DIS 8802/4 section 10-(1988)), each controller interfaces with a modem. In particular, controllers of the aforementioned communication systems could not share modems. Each controller interfaced with its corresponding modem.
Therefore, because of the relatively high cost of the modem, there is a need to reduce the number of modems employed in a communication system using the IEEE 802.4 standard. There is provided by the present invention an apparatus which permits multiple (token bus) controllers of the aforementioned communication systems to share a single modem. Further since the modem itself has an analog connection to a medium, there is created certain loads on the medium in terms of energy loading on the bus so that for a given system there is a limited number of ports for connecting the analog modem. By being able to share one analog modem across many digital controllers, there is effectively a multiplier placed in the system of how many devices can be connected. Without having to put repeaters in the analog line, a system is thereby created in which each analog port interfaces many digital controllers by sharing the modem.
A further feature of the present invention reduces the need to have fiber optic or coax cables to connect each device on the bus to every other device on the bus. In the present invention, because these signals are in the form of digital signals, printed wiring backplane or conventional multi-conductor ribbon cable and their associated connectors can be used for devices or modules within close proximity rather than having to utilize specialized fiber optic or coax connectors, and fiber optic or coax cables, respectively. Furthermore, the modem serves to isolate the devices or modules so that they can communicate with each other despite the presence of disabling faults elsewhere in the communications system. Also, for equipment within close proximity, the modems can be eliminated entirely (with a very minor modification), thereby reducing the cost, and still permitting the equipment to have the same interface as if the equipment were communicating remotely.